When applying a voltage < 1.2V the frequency will be under 434.650Mhz, and applying a voltage > 1.2V, the frequency will be above 434.650Mhz.

The NTX2's TXD input is normally driven directly by logic levels but will also accept analogue drive (e.g. 2-tone signalling). In this case it is recommended that TXD (pin 7) be DC-biased to 1.2V approx.

As read on UKHAS, the better “shift” seems to be around 500Hz.If we want 500Hz between 0 and 1, we shoud apply a difference of 0.25V on the TXD Pin.

FM deviation (peak) ±3 kHz

So we have 6KHz deviation for 3V range.

3V / 6000 = 0.0005V for 1Hz
500 × 0.0005V = 0.25V for 500Hz

So we can apply for exemple 2V for a 0 and 2.25V for a 1… (Or 1.25V and 1.5V etc..)

GPIO / Serial

According to UKHAS members , it’s not possible to use a regular GPIO output on the RaspberryPi to properly shift voltage to the NTX2 TXD. The reason is the Raspberry is not “real time”, so you will successfully change voltage (output of GPIO and voltage divider), but the timing will not be accurate enough to successfuly decode packets.

The “trick” is to use a serial connection.The reason is the serial circuit has it’s own crystal that will produce an accurate timing.

Dave Akerman use the internal embeded serial available on the Raspbery.As I wanted to keep the embedded serial, available for control, I used a Profilic PL2303 USB serial instead. This worked fine.

Note that the arduino as an accurate timing and don’t need the “serial” trick

Voltage divider

Output of the serial of the RaspberryPi is 3.3V level but output of Profilic PL2303 could be AFAIK 3.3V or 5V level.

In both cases, we will need a Voltage Divider to have the 0.25V shift within the range 0 - 3V.For exemple we could have:Low level (0V) -> 2.38 VHigh level (5V)-> 2.63 V

What is great, is the decoded packets can been sent by anyone, not only the owner of the balloon. This makes possible to send a balloon from UK, then track it for exemple to Ukraine with listeners in this area.

Installation

Informations on installation (compilation) of dl-fldigi are available on UKHAS wiki and are working fine on Debian too.

It seems PortAudio is using ALSA.. which is using PulseAudio…It’s also possible to directly use PulseAudio.

HAB mode

To start dl-fldigi in HAB mode which contain the list of registered flight, with all settings ready, use the folowing command:

./src/dl-fldigi --hab

Result of playing the wav file with totem with dl-fldigi in HAB mode.

In case of problem decoding with the sound card

/!\ During my fists tests, with several computers, several distros, the decoding with the sound card didn’t work.After few days of tests, I solved the problem by changing the default-sample-rate to 48000:

Introduction

SDR (Software Defined Radio) is a radio communication system where components that have been typically implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, etc.) are instead implemented by means of software on a personal computer.

The main advantage of SDR is the reduced cost.

In France (and many countries in Europe), we are free to use the 433MHz frequency (433,050 à 434,790 MHz).

This way we could be able to receive signal sent from the balloon, then decode the signal to get the coordinates etc...